To realize high fidelity audio reproduction it is of paramount importance for an amplifying device's bias current to be maintained at the “optimal bias current” where the highest audio fidelity is achieved. Deviations from said optimal bias current alter the device's gain and frequency response which negatively impact audio quality.
Amplifier bias current is influenced by many factors like ambient temperature change, instability of bias setting circuitry and device performance parameter drift (transconductance and offset) over time. As such it is desirable to establish a methodology to automatically and autonomously account for and correct for these effects.
An electronic device's bias current can be inferred by imposing said bias current across a resistor to create a sense voltage. Electronic control loops can then be used to monitor the low frequency (DC) components of the sense voltage and make circuit adjustments to maintain said optimal bias current. The important inference here is that the sense voltage DC component is a faithful representation of said device bias current. In other words it is assumed that a constant proportionality exists between the sense voltage DC component and said device bias current. This is called the proportionality inference.
When no perturbing signal, such as an audio signal, is passing through the device said proportionality inference is valid and the sense voltage DC component is guaranteed to represent said bias current. As such any sense voltage DC deviation represents a true said bias current shift which is accompanied by a control loop response serving to cancel the deviation reestablishing said optimal bias current.
There is a range of perturbing signal amplitudes increasing from “smaller” to “larger” where the introduction of said perturbing signal will not affect the sense voltage DC component and thus maintain said optimal bias current. This region is the termed the class A range. Outside this range, however, said perturbing signal amplitude becomes large enough to cause sense voltage distortion which in turn adds a sense voltage DC component. This distortion induced DC component is indistinguishable from a DC component due to said bias current shift and consequently the control loop works to remove it. In other words large perturbing signals cause distortion which forces a control loop correction which serves to adjust the device away from said optimal bias current. The region in which a perturbing signal becomes large enough to cause appreciable sense voltage distortion is called the class B range. It is important to note that this definition of class B differs somewhat from the strict industry interpretation. For the sake of brevity it is expedient to classify distortions arising from operation in class AB and true class B, as defined strictly in industry, into the comprehensive term “class B”. A control loop response resulting from a DC shift caused solely from entering class B operation is undesirable since said optimal bias is not maintained which negatively impacts audio quality.
Prior art approaches have attempted to minimize said class B sense voltage distortion by using limiting circuit elements (diodes) to limit the positive going portion of said sense voltage. This is called positive going limiting. One weakness of this approach is that the distortion introduced by negative going limiting is not considered or accounted for. Said negative going limiting adds distortion to said sense voltage which is neither systematic nor predicable, introducing DC components to the system which are incorrectly acted on by the control loop. The result is the control loop adjusts said bias current away from said optimal bias current value.
Another major failing of the prior art approach is that said positive going limiting depends on limiting circuit elements (like diodes) whose performance are strongly temperature dependent and not constant over time (drift). The result is that that said limiting circuit element can change said positive going limiting which causes an artificial DC component. A control loop response resulting from this DC component is undesirable since the device deviates from said optimal bias current.